Single Channel Properties of Heterotetrameric Mutant RyR1 Ion Channels Linked to Core Myopathies

Skeletal muscle excitation-contraction coupling involves activation of homotetrameric ryanodine receptor ion channels (RyR1s), resulting in the rapid release of Ca2+ from the sarcoplasmic reticulum. Previous work has shown that Ca2+ release is impaired by mutations in RyR1 linked to Central Core Disease and Multiple Minicore Disease. We studied the consequences of these mutations on RyR1 function, following their expression in human embryonic kidney 293 cells and incorporation in lipid bilayers. RyR1-G4898E, -G4898R, and -ΔV4926/I4927 mutants in the C-terminal pore region of RyR1 and N-terminal RyR1-R110W/L486V mutant all showed negligible Ca2+ permeation and loss of Ca2+-dependent channel activity but maintained reduced K+ conductances. Co-expression of wild type and mutant RyR1s resulted in Ca2+-dependent channel activities that exhibited intermediate Ca2+ selectivities compared with K+, which suggested the presence of tetrameric RyR1 complexes composed of wild type and mutant subunits. The number of wild-type subunits to maintain a functional heterotetrameric channel differed among the four RyR1 mutants. The results indicate that homozygous RyR1 mutations associated with core myopathies abolish or greatly reduce sarcoplasmic reticulum Ca2+ release during excitation-contraction coupling. They further suggest that in individuals, expressing wild type and mutant alleles, a substantial portion of RyR1 channels is able to release Ca2+ from sarcoplasmic reticulum